The Effect of Sulfate and Chloride Contamination on Zinc Phosphate Coating Weights
Acid pickles are commonly used prior to zinc phosphating to remove light scale, oxide and rust. Pickling produces a light etch to condition the surface prior to the zinc phosphating step. The most common acids used for pickling are hydrochloric and sulfuric. While this step is important, the rinsing step afterwards should not be ignored. When rinsing is not complete, chloride and sulfate ions will build in the phosphate bath and reach a level that will ultimately reduce the zinc phosphate coating weights produced. The following charts illustrate the reduction in zinc phosphate coating weight that can occur from chloride and sulfate contamination.
There are several steps that can be taken to reduce the drag in of contaminants.
  • Use counterflow rinse tanks when possible.
  • Increase the fresh water flow rate as needed.
  • Confirm rinse tanks are plumbed correctly. Fresh water inlet should be plumbed near the bottom of the tank. Outlet should be located on the opposite side at the top of the tank.
  • Rinses should be dumped and cleaned on a schedule. Increase frequency if needed.
  • Make sure immersion time is adequate in the rinse tanks.
  • Use air agitation in rinse tanks to increase rinsing efficiency.
  • There should be some hang time over tanks to decrease the amount of drag over.
  • When spray nozzles are used confirm they are not blocked or misaligned.      
Phos Dip 38 is a superior, production-proven zinc phosphate conversion coating for steel surfaces to retain lubricants and to facilitate cold drawing and forming of metals. A unique phosphate to nitrate ratio, plus increased metal accelerator produces a uniform, tight grained surface perfectly suited for optimum drawability. Phos Dip 38 reduces metal-to-metal contact, thereby increasing tool life, permitting faster drawing speeds, allowing greater reductions and deeper draws. The increased coating weights obtained using Phos Dip 38 will result in increased die life. Phos Dip 38 is the product of choice for shaped wire, cold heading and exceptionally difficult draws .

For more information please contact us:
Diana St. Jacques - Chief Chemist
  Dennis Breton - Surface Finishing R&D
Contact DuBois Chemicals
at (413) 452-2000